The LED device has matured greatly since the time of its inception over thirty years ago. The semiconductor technology enables an LED lighting device with orders of magnitude longer life than the traditional incandescent and fluorescent bulbs. To save energy and stop global warming, world-wide governments propose migrating away from the traditional incandescent and fluorescent bulbs. However, the cost of manufacturing LED lighting devices is still much higher than the traditional incandescent and fluorescent bulbs and therefore, becomes an obstacle to the migration. While the manufacturing cost of the LED lighting devices remains high, improving the performance of an LED driver; for example, achieving a high power factor correction value may be an alternative solution to reducing the cost and facilitating the migration from traditional lighting devices to LED lighting devices.
Early stage industry standard for industrial/home LED lighting drivers simply allows the LED current to be the same as the alternating current (AC) input current. FIG. 1 is an exemplary illustration 100 of an LED lighting driver with a same AC current flowing across an LED string and a coil in the prior art. In exemplary illustration 100, an AC current from a set-up circuit 101 may flow across an LED string 106 and a coil 108. Therefore, the current flowing through the LED string 106 is the same as the current flowing through the coil 108 and the AC current from the set-up circuit 101. However, in this prior art configuration, the AC current does not necessarily follow the AC voltage so as to achieve a high Power Factor level>0.9. When AC current varies in accordance with the AC line input to the set-up circuit 101, then such criteria serves to meet the PFC standard (PFC>0.9) for industrial/home lighting that most countries in the world adopted since the European Union instituted the IEC555 standard in early 1990s.
In the early stage technology of the LED lighting driver, although the average of the LED current may be constant, the instant LED current varies in a triangular shape, and behaves as a switching-type current flow in accordance with the AC input current. FIG. 2 is an exemplary illustration 200 of an LED current that varies over a wide range with peak and valley levels in the prior art. In exemplary illustration 200, although the average of the LED current may be constant, the instant LED current varies in accordance with the coil current peak and valley values and the LED current may also vary in a triangular shape, i.e., a switching-type current flow. Such architecture does not lead to high PFC because the AC current does not follow the sinusoidal AC voltage line.
The performance of the LED lighting driver improved in the past few years by adopting a true PFC stage prior to the LED current driver. FIG. 3 is an exemplary illustration 300 of another LED lighting driver with separated AC current and LED current in the prior art. The exemplary illustration 300 may comprise a set-up circuit 301, a boost PFC regulator 305, a first coil 308, a first transistor 310, a first diode 309, a first capacitor 307, an LED string 321, a second capacitor 320, a second coil 322, a second diode 323, a second transistor 324 and an LED control 325. The boost PFC regulator 305 may be implemented to regulate a direct current (DC) level voltage at the anode of the LED string 321. In some embodiment, the DC level voltage may refer to a VPFC level voltage of approximate 400v. The VPFC level voltage may provide higher tolerance to current variation than the AC line input, and thus, current flowing through the LED string 321 may no longer be sensitive to the AC line input and the performance of the LED string may be increased. This approach increased the performance of the LED Lighting as a fixed level voltage (VPFC) inputted into the LED string is no longer sensitive to a varying AC input. Meanwhile, the industry increased the PFC standard to be higher than 0.9 and closer to 0.98.
Recent research shows that the current flowing through LEDs may be forced to true constant rather than varying around a fixed average value. Further, dimming capability when the LED current becomes constant improves the LED performance by at least one order of magnitude. One example of the above approach is described in European Publication EP 2315497A1. However, in EP 2315497A1, the LED string cannot be connected directly to the VPFC level voltage. Further, U.S. Pat. No. 7,157,809 B2 underlines the problems to regulate current in non-linear loads and provides a general solution. U.S. Pat. No. 7,157,809 B2 provides a regulated load/LED current as long as the current source is maintained with sufficient headroom from the current source drain to the current source terminal. Satisfying the headroom requirement guarantees the load/LED current is constant and independent from the variation of the output voltage. LED lighting devices may benefit from the above noted approach because the PFC stage implemented prior to the load provides a constant level voltage to the LED string and enables the VPFC to be independent from the AC input. FIG. 4 is an exemplary illustration 400 of a boost converter with higher output voltage than input voltage in the prior art. The exemplary illustration 400 of a boost converter may comprise a set-up circuit 401, a boost PFC regulator 405, a coil 408, a transistor 410, a diode 409 and a capacitor 407. The boost converter allows the output voltage to be always higher than the input voltage. Under the circumstances of the universal range discussed above, the output voltage of the boost converter may be higher than 276 AC, which has been standardized to as high as 400v DC. The boost converter in exemplary illustration 400 has particular advantages in the LED lighting market because the AC current is always continuous and makes it easier to follow the AC line voltage. However, a disadvantage of the exemplary illustration 400 is that there is no intrinsic current limit and therefore additional circuitry may be required to limit the inrush current during startup.
The industry has accustomed to so-called universal line, where some countries operate at 85 AC while others operate at 276 AC. This is fundamentally 100 AC−15%. and 240 AC+15%. Such a wide range of AC levels must be met via a universal line power supply, and an LED lighting device has to be designed to fit the requirement of the wide range. Under the circumstances of the universal range discussed above, the output voltage of the boost converter may be higher than 276 AC, which is standardized to as high as 400v direct current (DC). As illustrated in FIG. 5, the 400v DC level, i.e., VPFC level is applied to the LED string. However, such applications require a large number of LEDs of approximately 130 LED diodes, as each LED diode has approximately 3v drop at a room temperature.